Well cementing is a process used in penetrating subterranean formations that produce oil and gas. In well cementing, a well bore is drilled while a drilling fluid is circulated through the well bore. The circulation of the drilling fluid is then terminated, and a string of pipe, e.g., casing, is run in the well bore. The drilling fluid in the well bore is conditioned by circulating it downwardly through the interior of the pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the well bore. Next, primary cementing is typically performed whereby a slurry of cement in water is placed in the annulus and permitted to set, i.e., harden into a solid mass, to thereby attach the string of pipe to the walls of the well bore and seal the annulus. Subsequent secondary cementing operations, i.e., any cementing operation after the primary cementing operation, may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
One problem commonly encountered during primary and secondary cementing operations is the movement of water from the subterranean formation into the well bore, resulting in the influx of water into cement slurries that have been placed in the well bore. In particular, the influx of water occurs during a transition phase in which the cement slurry changes from a true hydraulic fluid to a highly viscous mass showing some solid characteristics. When first placed in the well bore, the cement slurry acts as a true liquid and thus transmits hydrostatic pressure. During the transition phase, certain events occur that cause the cement slurry to lose its ability to transmit hydrostatic pressure. One of those events is the loss of fluid from the slurry to the subterranean zone. Another event is the development of static gel strength, i.e., stiffness, in the slurry. When the pressure exerted on the formation by the cement slurry falls below the pressure of the water in the formation, the water begins to flow into and through the cement slurry. This influx of water can occur during dynamic and static states of the cement slurry.
As a result of water influxes into and crossbows through the cement slurry, flow channels form therein that remain after the cement slurry has completely set. Those flow channels allow the water to flow from one subterranean zone to another such that zonal isolation is no longer achieved. Further, the water intermixes with and dilutes the cement slurry, causing deterioration of the cement properties such as its density, its final compressive strength, and its rheology. As such, the water adversely affects the integrity of the cement. Secondary cementing is often used to repair the lost integrity of the cement placed in the annulus during primary cementing. However, the cement slurries employed during secondary cementing also become deteriorated due to the continued influx of water. The secondary cementing operation therefore may fail to perform as designed in forming a sealing or blocking mechanism.
Repairing the deteriorated cement can be both time consuming and costly. As such, various chemicals have been used to attempt to prevent the influx of water into the cement. For example, silicates such as sodium silicate have been added to the cement or injected ahead of the cement to react with it, hydrate it, and cause it to set more quickly. Other chemicals have been added to the cement to increase its viscosity and make it less permeable to water. However, such chemicals often become diluted and dispersed before they can effectively inhibit the influx of water. It would therefore be desirable to develop improved processes for protecting cement slurries against deterioration caused by the influx of water.